This section provides background information related to the present disclosure which is not necessarily prior art. This section also provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.
As it is known, steel structures are vulnerable under fire hazards; external passive fire protection is thus needed to achieve sufficient fire resistance. Spray-applied fire-resistive materials (SFRM) are one of the most commonly used passive fire protection material for steel structures in North America. SFRM are predominately cementitious (many are Portland cement-based) lightweight plaster materials that are sprayed onto the steel surface. They have very low thermal conductivity so that they are highly effective in delaying the temperature rise in the steel and protect the steel structure against fire related failures. Apart from the central function as a fire-protection material, SFRM offer many additional advantages including low self-weight, ease of construction facilitated by sprayability and most importantly, cost effectiveness; therefore, SFRM are widely used to protect steel structures in North America.
Despite all the advantages, the performance of SFRM also naturally depends on their durability characteristics (the ability to stay on the steel surface), which are often called into question. Studies have shown that SFRM could easily delaminate or get damaged during extreme loading events, including earthquakes or impacts. Under normal service life, regular mechanical maintenance work or construction could also disturb and damage the existing SFRM and repair is then required. Failure to restore the fire protection in a timely manner could lead to a reduction in the fire resistance of the steel structures. This problem becomes more prominent under multi-hazards such as post-earthquake/impact fire. The lack of durability is then recognized as the major issue associated with conventional SFRM.
Adhesion and cohesion are two major durability characteristics for SFRM. Adhesion refers to the interfacial bond between SFRM and steel substrate and sometimes could be enhanced through preparation of the steel surface and applying external bonding agent. Cohesion refers to the material's resistance to delamination (often observed as a thin layer of SFRM material still attached to the steel after the two bulk material separated in a shear type failure) and fracture. Cohesion is an intrinsic material property that is largely dependent on the strength and ductility of the material itself. SFRM are inherently brittle material with very low strength, especially tensile strength; therefore conventional SFRM mainly rely on adhesion to maintain the integrity of the fire protection. Recent study demonstrated that even with adhesion enhanced by external interfacial bonding agent, delamination of SFRM due to poor cohesive property was still observed when subject to impact load. The poor cohesive performance represents the major bottleneck for conventional SFRM.
To overcome the inherently brittle and low cohesive property of SFRM, recent effort has been made to adopt Engineered Cementitious Composites (ECC) technology into the design of SFRM according to the principles of the present teachings. ECC is a class of High Performance Fiber Reinforced Cementitious Composites (HPFRCC) that has been developed over the last decade as a ductile alternative to the conventional concrete. Unlike the conventional cement-based material, ECC exhibits metal-like pseudo strain-hardening behavior with strain capacity up to 3-5% under uniaxial tension. Such high tensile ductility is reached by forming multiple fine cracks (typically less than 100 μm wide) along the specimen. The fracture resistance of ECC is considered similar to aluminum. With these desirable characteristics, ECC possesses inherently high cohesive property to conventional SFRM. ECC developed for structural applications, however, do not possess the necessary thermal characteristics required for fire-protection purpose. Adopting the micromechanics-based design methodology underlying ECC technology in combination of microstructural tailoring for macro-thermal property control, spray-applied fire-resistive ECC (SFR-ECC) has been developed as a durable alternative to the conventional SFRM.
The present disclosure is a comprehensive introduction of SFR-ECC. The material composition, durability properties (under both static and high rate load), and functionality properties (thermal conductivity and sprayability) are further presented. The cohesive characteristics of SFR-ECC provide extra durability mechanisms in addition to interfacial adhesion, and its sprayability allows versatile construction applications of SFR-ECC, as will be discussed herein.
An objective of the present teachings is to develop a new class of high-performance fiber reinforced cement-based composites (HPFRCC) that possess very low thermal conductivity and high tensile ductility. Presently there are cement-based materials that separately possess very low thermal conductivity (conventional spray-applied fire-resistive materials) or high tensile ductility (engineered cementitious composites); however, there is no material that possesses both properties. The present teachings provide a newly developed composite material with composition that leads to a combination of low thermal conductivity and high ductility. The material claimed herein presents a unique opportunity of more durable fire protection for steel structures and enhanced safety of steel structures under multi-hazard such as impact/earthquakes followed by fire.
In other words, according to the principles of the present teachings, a cement-based fire-resistive material with high tensile ductility and low thermal conductivity is provided. This invention represents the culmination of two cement-based material, namely those of very low thermal conductivity and those of very high tensile ductility into a single composite system. The combination of such properties are achieved by judicious selection of the lightweight aggregate (of small size and smooth shape) and fibers (type, aspect ratio, volume content) under the guidance of heat transfer theory and micromechanics analysis. In doing so, the new Fire-Resistive Engineered Cementitious Composite (FR-ECC) exhibits greatly enhanced tensile ductility over conventional cement-based Spray-applied Fire-resistive Materials (SFRMs). This makes it a candidate material as durable fireproofing material for steel structures when impact or earthquake loads are of concern or to generally improve the durability of the fire protection. This invention can be used as insulation board system or as sprayed-on fireproofing material.
Due to the high durability of SFR-ECC, it is feasible to spray applied this material onto steel elements prior to on-site assembly, making the factory spray quality control better compared with on-site spraying, and speeding up the steel building construction process as a result. This feature is not feasible with conventional SFRMs since they will crack and delaminate from the steel members during transport to and assembly on site.
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.